In many subsea fluid extraction wells, equipment such as rotary motors are positioned at the sea floor or at a down-hole location to control the production and delivery of hydrocarbons to the sea surface. Much of this equipment is subject to wear, and therefore needs to be repaired or replaced periodically. Since the condition in which this equipment operates varies greatly from application to application, and since every situation is unique, proper maintenance intervals can be difficult to predict. When proper maintenance intervals are underestimated, equipment failure and an associated emergency halt in production often results.
To help assess the need for maintenance, various sensor assemblies have been provided to monitor indirect parameters related to equipment health such as temperature, pressure and flow rates. In one example application, accelerometers mounted to a motor case indirectly monitor a motor shaft by monitoring movement of the motor case that may have been caused by an unbalance or vibration of the motor shaft. Often, this method is not very accurate. At least since the environmental conditions encountered in subsea fluid extraction wells are generally unique for each application, a problematic accelerometer data pattern may not be immediately recognized. Also, since the motor case is generally much heavier than motor shaft, small variations in the movement of the motor shaft induce even smaller variations in the movement of the motor case. Thus, in some instances, problematic movements of the motor shaft are not recognized in the early stages and persist until the movements become more pronounced. This delay can be associated with poor diagnostic information.
One application in which indirect monitoring can provide poor diagnostic information is in a subsea booster pump that employs fluid film bearings. Fluid film bearings generally support their loads on a thin layer of liquid or gas, and are frequently used in high load or high speed applications where ball bearings would wear quickly or cause excessive noise or vibration. In some instances, fluid film bearings permit a motor shaft to rotate in an off-center or elliptical orbit when properly operating, and these orbits can be difficult to distinguish from problematic rotational patterns using indirect methods such as detecting a problematic acoustic signature.
Accordingly, recognized is the need for directly monitoring a motor shaft for assessing the health of the motor used in subsea applications. Direct monitoring of a motor shaft can include monitoring dynamic motion parameters such as vibrational amplitude, frequency and phase angle, as well as static, quasi-static or steady state position measurements such as steady state eccentricity position, axial thrust position and eccentricity slow roll. Other parameters of a motor shaft can be directly monitored to facilitate assessing motor health. Also recognized is the need for providing such a sensor assembly that can be readily installed into existing motor assemblies to directly monitor the motor shaft in a variety of configurations and orientations.